1. Field of the Invention
The present invention relates generally to printed circuit boards, and more particularly to approaches for connecting one printed circuit board to another.
2. Disclosure Information
Printed circuit boards (PCBs) are available in a variety of different types. Some PCBs are rigid, such as those having a substrate made of alumina or FR-4 glass/epoxy laminate, while others are relatively flexible (i.e., "flex circuits"), such as those having a substrate made of polyimide, polyester, and the like. Sometimes it is necessary to connect one PCB to another; that is, to electrically connect the circuit traces of a first PCB to corresponding circuit traces on second PCB. This board-to-board connection can be made either directly or indirectly. Indirect connection is the more common approach, which utilizes one or more connectors as an intermediary between the two boards. For example, as illustrated in FIG. 1, one board 10 might have a through-hole type connector 90 with male pins 92 thereon, while the other board 20 might have a surface mount type connector 94 with female receiving sockets 96 therein. The connectors 90/94 not only electrically connect the respective traces 14/24 together, but also acts as a mechanical strain relief against shear or flexure of one board with respect to the other. On the other hand, direct connection approaches do not rely on an intermediate connector, and are usually limited to cases where one or both of the boards is a flex circuit. (Directly connecting two rigid boards together without an intermediate connector is typically not practical, because the solder joints between the boards' adjoined traces cannot withstand much shear or flexure of one board with respect to the other.)
The typical direct connection approach for connecting a first (flex circuit) PCB to a second (flex or rigid) PCB involves the steps of (1) arranging the respective traces in correspondingly matching arrays at the edge of each board, (2) applying solder paste, conductive adhesive, or the like to the ends of the traces where connection is to occur, (3) overlapping the edges of the PCBs and placing the respective trace arrays in direct contact with one another, (4) applying pressure and heat through a mandrel or hot bar to the overlapped area so as to melt the solder paste, and (5) removing the heat and pressure so as to form solid solder joints connecting each trace of the first PCB with its counterpart trace on the other PCB. This is known as the "hot bar solder reflow" process. In order to effectively reflow the solder, the temperature of the hot bar or mandrel is typically maintained at about 280-300.degree. C.
When the more expensive polyimide is used as the flex circuit substrate material, the hot bar process may be used, since the glass transition temperature of polyimide is typically about 350.degree. C. However, when the less expensive polyester material is used as the substrate, hot bar processing cannot be used, since the glass transition temperature of polyester is about 70.degree. C. In fact, use of the hot bar process would melt, distort, and severely damage a flex circuit having a substrate made of polyester. Thus, hot bar and other direct connection processes typically cannot be used with polyester flex substrates, thus requiring the use of connectors and an indirect connection approach. This presents a dilemma, because although polyester is more attractive than polyimide from a materials cost standpoint, the cost of the connectors and associated processing often offsets much of the savings that polyester seems to provide in the first place. It would be desirable, therefore, to provide an alternative to hot bar processing which permits direct, connectorless connection between one or more flex circuits made of polyester substrate.